By Taffy Brodesser-AknerNEW YORK TIMES NEWS SERVICE • Wednesday December 26, 2012 7:23 AM

In the opening episodes of
Gossip Girl this season, the gang was once again on the trail of a missing Serena. (She
has disappeared before.)

She was keeping secret the fact that she had slept with a boyfriend of Blair’s (not the first
time). Dan was writing a tell-all (again). And Rufus was making waffles. (It’s what he does.) If it
seems as if the show had visited this territory before, it had.

But the
Gossip Girl writers hadn’t run out of ideas. No, they were creating mirror-image bookends
to the show’s first seasons, demonstrating how the characters had come full circle. They were doing
the part- triumphant, part-sad job of wrapping up
Gossip Girl.

After six seasons, the show went dark this month — one of several notable series, including
The Office and
30 Rock, that are ending runs this TV season.

Shows go off the air for various reasons: tanking ratings, tanking ratings and — of course —
tanking ratings.But sometimes — rarely, even — shows go off the air not because of advertising
concerns or profitability but because the people who write and produce them decide that it’s
time.

That’s not to say these shows are leaving as the critical and ratings darlings they once were;
critics have faulted all these shows for being stale. Renegotiating with actors, writers and
producers often creates too big a dent in profitability. When contracts are up, a show is lucky if
the star — now popular — is still willing to come to the table.

Whatever the factors, the producers must face a big question: When do you call it quits?

For
Gossip Girl, the answer was easy. “Shows like these aren’t designed to run forever,” said
Josh Schwartz, the series co-creator.

“There comes a point creatively where it feels like having an endgame is a terrific way to
energize a show late in its run.”

Likewise, when
The Office began its ninth season, it seemed like a show that could go on forever — which
was, in fact, a possibility.

“We could keep the show going for many years and keep refreshing it with new cast members,” said
Greg Daniels, one of the show’s creators and executive producers. “We hit a fork in the road this
season, trying to decide if we’d go that way or if we’d end it now with as many of the original
cast members as we could.”

Exhaustion was also partly to blame for the curtain falling on
30 Rock, which will end on Jan. 31. But a bigger reason was the eagerness of the staff to
pursue other projects — especially the creator, Tina Fey, who stars as Liz Lemon.

“People want to move on and have to move on,” said Robert Carlock, an executive producer of
30 Rock. “A show has a half-life, an isotopic decay, and you have to be sensitive to that.
The thing that kept us on the air was that we tried to make it very good. The last thing that Tina
and I wanted to have happen was it to stop being good.”